1. Field of the Invention
This invention pertains to municipal solid waste combustors (MWCs) and, by extension, to all steam power generation boilers and other furnaces. It involves handling and disposal by thermal decomposition of waste sludges of variable solids content, including municipal sewage sludge, paper mill sludge, and other industrial sludges, in MWC power boilers and other furnaces which burn municipal solid waste or other fuels.
2. Background of the Related Art
As the options available for communities and industries for sludge disposal decline, a new cost-effective, environmentally sound method of disposal has become desperately needed. In the 1950s and 1960s, sludges were often incinerated in multiple hearth furnaces. However, most of those installations experienced poor combustion and produced extremely adverse air emissions. Accordingly, this approach was largely abandoned in favor of non-combustion disposal technologies.
Co-combustion of sludge, and preferably high carbon content sludge, with other fuels has remained a goal for many years. One approach, discussed repeatedly in technical literature, was to design external thermal drying systems for pre-drying sludge prior to injection into a furnace, so that drying sludge inside the furnace would not be necessary. However, this technology was expensive and even dangerous, because of potential health hazards related to inhalation of fugitive dust and dust explosions.
Other approaches described below involved injection of wet sludge into modern combustion processes. These approaches failed to achieve commercial acceptance of their technologies because of either limited applicability, economic disadvantages or technological disadvantages.
Komline et al. in U.S. Pat. No. 3,322,079 noted the synergy in co-combustion of sludge and municipal solid waste (MSW), using high temperature gases from combustion of MSW to dry sludge injected into the furnace by a rotary centrifugal atomizer. However, operating costs of such a system are high and the rotary centrifugal atomizer disclosed is difficult to reliably maintain because of plugging and wear. The rotary atomizer discharges a significant fraction of large particles that do not burn completely, and deposition of material on boiler internals becomes a problem.
Dingwell in U.S. Pat. No. 3,838,651 used steam to atomize waste oil in a burner designed to extend into a combustion chamber. However, this invention is limited in scope to waste oils in old-fashioned incinerators, which were not regulated as stringently at the time of invention. Partial combustion was acceptable and regulatory compliance was not identified as a goal. While steam was used for atomization, non-oil, i.e. aqueous, sludges were not considered.
Pan in U.S. Pat. No. 3,903,813 developed a device for injecting steam-atomized sludge into a combustion chamber in close proximity to an oil or gas burner. The injection device allows for mixing of a pressurized fluid and sludge in the same pipe, with the mixture exiting a restrictive orifice at the end of the injection pipe, such that the mixture is flashed to atmospheric pressure inside the furnace in the presence of oil or gas. Co-combustion with MSW and other renewable and fossil fuels is not considered and the device is intended for use primarily with sludges having solids content equal to or less than 5%. The point of sludge injection is limited to an area below an oil or gas burner flame. This device is intended for incineration of sludge only in the presence of an oil or gas burner.
Carpenter in U.S. Pat. No. 5,284,405 disclosed a method for entraining sludge particles in a stream of compressed combustion air, but the invention is limited to combustion within a rotary cement kiln. No attempt is made to burn sludge in suspension and regulatory compliance is not addressed.
Goff et al. in U.S. Pat. Nos. 5,052,310 and 5,405,537 established a sludge injection system for MWC's using oxygen-enriched air both to atomize sludge being sprayed into a furnace through a nozzle and to offset the loss of boiler efficiency from moisture in the sludge. However, this requires expensive construction of an oxygen-producing plant adjacent to an owner's core production facilities or purchase of large quantities of bottled oxygen. Furthermore, the invention is limited to MWC's only and may require modification to existing combustion air control systems.
Mole in U.S. Pat. No. 5,531,169 injects liquid waste material, primarily contaminated acid, immediately adjacent to a primary Fuel burner. The primary purpose of the device is to dissociate acid molecules. The device is intended for air as opposed to steam atomization and the invention is directed to liquid hazardous waste disposal as opposed to sludge disposal and non-hazardous municipal solid waste combustion and conventional power generation.
Guibelin in U.S. Pat. No. 5,544,598 developed a nozzle for disposal of slurry, such as pasty or fatty wastes, by spraying onto burning municipal solid waste. However, this device does not involve sludge, and it is not designed for atomizing particles to burn in suspension.
An objective of the present invention is to provide a sludge receiving and treatment system that is capable of handling sludges with a wide range of characteristics (including variable moisture and solids content) and delivering a consistent flow of sludge to the injection system. Previous systems were not designed for and did not address variable sludge consistencies and solids content.
It is also a goal of the present invention to avoid the material handling difficulties typically encountered with sludge having solids contents of 15% and higher.
A further purpose of the present invention is to maximize suspension burning of sludge particles, in order to virtually eliminate the need to burn larger sludge particles.
A still further goal of the present invention is to utilize wastewater, other industrial process liquid wastes, air pollution control slurries, or mixtures thereof, in a sludge pre-combustion treatment process.